I. Field of the Invention
This invention relates to methods of preparing whiskers of silicon carbide and other materials e.g. A1N, sialons, Si.sub.3 N.sub.4, etc.
II. Description of the Prior Art
Whisker-form products are of increasing interest these days in view of the ever expanding requirement for new materials. For example, whisker-like crystals of silicon carbide are extremely hard and exhibit great tensile strength, and so are currently in great demand for a variety of purposes, e.g. as reinforcements for ceramic or metal matrix composites.
A variety of routes for the formation of silicon carbide whiskers and whiskers of other materials have been investigated, including both gas-phase reactions and solid-phase reactions. However, the yields tend to be poor, which makes the product expensive, and the whiskers are often contaminated with particulates, with unreacted starting material or with material from the reaction environment.
When silicon carbide whiskers are produced by heating silica to high temperature in the presence of carbon, the product is mainly powder- or lump-form silicon carbide. The growth of whiskers can be encouraged by providing a catalyst for whisker growth and voids in the carbon mass in which the whiskers can grow. However, the product inevitably contains a significant amount of powder-form silicon carbide which is troublesome to remove from the desired whiskers.
Improvements in this procedure have recently been reported by Tokai Carbon KK (Japanese Patent Application 59 9519 published on Mar. 2, 1984) and by Asahi Chemical Industry Co. Ltd. (Japanese Patent Application 58 223698 published on Dec. 26, 1983). The Tokai process involves reacting SiO.sub.2 obtained from chaff ash and carbon black under an inert gas, e.g. argon, at high temperature to produce volatile SiO in a first reaction zone and then causing the SiO to react with porous or fibrous carbon material in a second reaction zone. While this process reputedly produces .beta.-SiC whiskers of high purity, the yields of the whiskers are very low and this may be due to the formation of SiC in the first reaction zone as well as the desired gaseous SiO.
The Asahi process is similar to the Tokai process except that it is designed to produce fibres made up of SiC whiskers and it employs a yttrium compound as a catalyst for the whisker formation. Despite the use of the catalysts, the yield of whiskers is quite low and the purity of the resulting SiC is not stated.
In view of these problems, the Tokai and Asahi processes are not entirely satisfactory and there is a need for an alternative method of producing SiC whiskers of high purity in good yields and at relatively low cost.